Field of the Invention
The invention relates to a ceramic substrate suitable for: circuit boards for use in various industrial devices such as motors, vehicle mounted devices such as engine control units, or home electric appliances such as refrigerators, air conditioners, and televisions; circuit boards for use in various electronic devices including mobile communications devices such as cellular phones and smart phones; and high frequency parts such as high frequency filters and high frequency antennas. The invention also relates to a method for producing such a ceramic substrate.
Description of the Related Art
In recent years, semiconductor LSIs, chip parts, and other parts have decreased in size, thickness, and inter-terminal pitch. Therefore, ceramic substrates on which these parts are to be mounted have also been required to be smaller, thinner, and higher in precision. Conventionally, alumina is mainly used as a dielectric ceramic for forming ceramic substrates. Since the sintering temperature for alumina is as high as 1,300 to 1,600° C., high melting point metals such as W and Mo are used as electrode materials for wiring, such as transmission lines, on ceramic substrates. Unfortunately, W and Mo have high electric resistivity and high loss in the high frequency region and thus cannot be used for high frequency parts or parts requiring low resistance wiring. Thus, low resistance metals such as Cu, Ag, and Au have come into use as electrode materials, and dielectric ceramics capable of being sintered at a low temperature of about 800 to 1,000° C. simultaneously with low melting point metals such as Cu, Ag, and Au (low temperature co-fired ceramics) have come into use.
On the other hand, parts are downsized by forming built-in passive elements such as inductors and capacitors in ceramic substrates and by forming built-in transmission lines of the electrode materials on ceramic substrates. When built-in capacitors are formed, the use of dielectric ceramics with a higher dielectric constant allows the capacitors to have relatively higher capacitance. However, the use of dielectric ceramics with a higher dielectric constant can cause delay of high frequency signal transmission in transmission lines and can often increase crosstalk due to the stray capacitance generated between lines. Thus, Patent Document 1 proposes that a green sheet for forming a low dielectric constant ceramic layer and a green sheet for forming a high dielectric constant ceramic layer are stacked and simultaneously fired to form a low dielectric constant ceramic layer as a transmission line and to form a high dielectric constant ceramic layer as a capacitor dielectric. Patent Document 2 also discloses that a low temperature co-fired ceramic is used to form a ceramic substrate made of a composite of ceramic layers with different dielectric constants (hereinafter, such a ceramic substrate will also be referred to as a composite ceramic substrate).
For example, Patent Document 1 discloses that a dielectric ceramic composed mainly of La2O3.2TiO2 is used to form the high dielectric constant ceramic layer while a dielectric ceramic composed mainly of Al2O3 is used to form the low dielectric constant ceramic layer. Patent Document 2 discloses that a Pb-based perovskite Type dielectric ceramic is used to form the high dielectric constant ceramic layer while a BaO—Al2O3—SiO2 dielectric ceramic is used to form the low dielectric constant ceramic layer. As mentioned above, dielectric ceramics with significantly different compositions have been principally used, depending on dielectric constant, for composite ceramic substrates.
In order for dielectric ceramics with significantly different compositions to be bonded strongly without delamination or cracking by co-firing, it is necessary to select dielectric ceramics whose thermal expansion coefficients or contraction behaviors during firing are close to each other, and it is also necessary to take into account how to suppress fluctuations in dielectric characteristics such as dielectric constant at the bonded interface between the dielectric ceramics, which would otherwise be caused by mutual diffusion of components.
Patent Document 3 discloses that when first and second insulating layers including dielectric ceramics with different dielectric constants are used to form a composite ceramic substrate, the presence of (Mg,Ti)2(BO3)O as a common crystal phase in the dielectric ceramics with different dielectric constants allows them to have close contraction behaviors during firing and makes it possible to suppress cracking, delamination, or warpage in the composite ceramic substrate.